The M1 Abrams Tank is a ubiquitous symbol of American military armored vehicles. The treads that carry its bulk across the field of battle aren't the only method of carriage possible that fills the same role though.
Between 1916 and 1917, the Holt Manufacturing Company (now Catterpillar Inc.), created the Steam Wheel Tank for the U.S. Army. Its design involved two 8-foot-by-3-foot wheels at the front of the substructure on both the right and left sides, with a third wheel at the rear used for steering. The main weapon was a 75 mm Howitzer, with secondary Browning 50-caliber machine guns on either side of the main fighting compartment.
The tank's design allowed for a crew of six men (top-down view here). Two Doble 75hp steam engines powered this behemoth, with each front wheel driven by its own engine. From the Landships website:
"The engines were mounted horizontally, each running its own wheel, drive was taken from the pistons to driving and roller pinions, which engaged internal gears fixed to the front wheels. Boilers were carried behind each engine; ventilation was achieved by using exhaust louvers in the rear of the vehicle, in addition to a fan driven by steam from the boilers, and a radiator which acted as a condenser for the engines. Exhaust was through two small openings behind the main housing near the rear of the vehicle."
Some reports from the Aberdeen Proving Ground in Maryland said that the wheels would get bogged down under the tank's 17-ton weight. According to Landships, the tank did get stuck ... but only until enough pressure built up in the kerosene-fired boilers. Once that happened, the tank was able to get moving again.
Perhaps the most interesting thing about this tank is that the two 8-foot diameter main wheels (3-feet wide, made out of pressed steel) weren't a special military construction. They came standard on the Holt farming combines of the same period.
The Holt Steam Tank was never put into mass production. Electric starters soon became prevalent on internal combustion engine vehicles, and the steam tank, like the steam car, ended up relegated to the proverbial dustbin of history.